In general a microelectromechanical system (MEMS) or a micromechanical system (MMS) can be integrated in small devices or systems which combine electrical and mechanical components with one another. By way of example, the term “micromechanics”, related to micromechanical parts, can be used to describe small integrated devices or systems which include one or a plurality of micromechanical elements and possibly, but not necessarily, electrical components and/or electronic components.
In general a microelectromechanical system can be used to provide for example an electromechanical transducer, e.g. actuator or sensor. An MMS may include a deflectable structure, such as e.g. a membrane or a cantilever. Used as a drive, a microelectromechanical system (MEMS) may include one or a plurality of MMS whose deflectable structure can be electrically deflected. Used as a sensor (e.g. microphone), an MEMS can provide an electrical signal in reaction to a deflection (also referred to as stroke) of the deflectable structure of the MMS.
A microstructure such as, for example, a membrane in a microtransducer (for example microphone or microloudspeaker) or a cantilever in an atomic force microscope (AFM) can have stringent requirements, in particular in respect of the bending properties and/or the deflection behavior and the dynamic behavior under resonance conditions depending on the respective application.
Both the electrical and the mechanical requirements made of membrane-based sensors (e.g. microphones) increase with each successor generation and/or with the passage of time. By way of example, ever smaller sensors are demanded which tolerate higher loudness levels or the sound pressure level associated therewith, have a greater robustness and provide a higher signal-to-noise ratio (SNR).
As an assessment criterion for evaluating the quality and/or marketability of a sensor, both the SNR (Signal to Noise Ratio) value and the acoustic overload (also referred to as AOL) are of importance. A higher SNR enables the useful signal to be clearly demarcated from the background noise. At the same time the sensor should be able to clearly pick up a high sound pressure level (e.g. in concerts), without sound distortions occurring (also referred to as THD or total harmonic distortion). The electrical performance may be closely linked to the mechanical properties of the micromechanical structure.
By way of example, stringent requirements are made in respect of the mechanical robustness of a sensor for use in devices (e.g. in Smartphones, Smartwatches, Tablet PCs, Notebooks, Head-Sets or other everyday items of use) which are exposed both to mechanical loads (vibrations, fall) and to external environmental influences (dust, water, etc.). The higher the mass of the terminal device, the higher the mechanical load (illustratively pressure surge) can be for the microphone in the event of a so-called fall or impact of the device. In order to increase the robustness of a membrane-based sensor, conventionally various concepts are implemented, but they cannot simultaneously cover all requirements on account of the material systems available.
In one conventional concept, satisfying the requirement in respect of robustness, which requirement is (greatly) dependent on the mass of the terminal device, is controlled by way of the membrane thickness. The thicker the membrane, the more robust it is in the event of a dynamic pressure surge. However, this forces a compromise since, as the membrane thickness increases, the mechanical properties such as restoring behavior and sensitivity are also altered. In order to satisfy the requirements in respect of the restoring stress range while simultaneously complying with the sensitivity, a soft membrane composed of polysilicon (also referred to as poly-Si) is used, therefore, which is made additionally softer by its being implanted to a greater level. The smaller the membrane diameter, the lower the limit for implantation at which a degeneration of the poly-Si occurs, which leads to an additional induction of stress, with the result that the membrane buckles (also referred to as compressive membrane buckling at the implantation limit). The sensor thus becomes unusable, such that the maximum implantation of a small membrane diameter (e.g. less than 800 micrometers) is defined by the implantation limit.
Since there is a constantly increasingly endeavor to attain ever smaller devices, on account of the associated reduction of the housing sizes of the terminal devices it is necessary likewise to reduce the size of the MEMS and thus the membrane diameter. In order to retain the functionality of the MEMS it is necessary to comply with a sufficient distance from the implantation limit during implantation. Given a predefined restoring stress range, this can only be compensated for by reducing the membrane thickness, but this in turn results in a lower robustness.
In the case of small membrane diameters it is therefore conventionally accepted that the sensor has a low life expectancy or a low sensitivity.
In an alternative concept, geometric modifications that influence the mechanical properties are conventionally implemented on the MEMS. By way of example, so-called ventilation flaps are incorporated into the membrane, which attenuate a pressure surge and thus compensate for a lower robustness of the membrane. However, the ventilation flaps require a precise setting of the so-called corner frequency (or cut-off frequency). Alternatively, so-called corrugation rings are incorporated into the membrane, at which the membrane is corrugated. The implementation of the corrugation rings and/or of the ventilation flaps increases the production costs, generates additional stress points in the membrane and/or counterelectrode and increases the risk of the membrane “sticking” to other component parts, e.g. in the case of a double electrode configuration (also referred to as dual backplate arrangement).